Push: Ensure ref updates are processed in input order

Push: Ensure ref updates are processed in input order

Various places on the client side of the push were creating unordered
maps and sets of ref names, resulting in ReceivePack processing commands
in an order other than what the client provided. This is normally not
problematic for clients, who don't typically care about the order in
which ref updates are applied to the storage layer.

However, it does make it difficult to write deterministic tests of
ReceivePack or hooks whose output depends on the order in which commands
are processed, for example if informational per-ref messages are written
to a sideband.[1]

Add a test that ensures the ordering of commands both internally in
ReceivePack and in the output PushResult.

[1] Real-world example:
    https://gerrit-review.googlesource.com/c/gerrit/+/171871/1/javatests/com/google/gerrit/acceptance/git/PushPermissionsIT.java#149

Change-Id: I7f1254b4ebf202d4dcfc8e59d7120427542d0d9e